>They do start the season as solitary bees, in the sense that they have to
feed themselves and their brood to start the colony.

Exactly!  Bumblebees are solitary bees for the majority of the queen's
life, and social bees during the colonial period while flowers are abundant.

>As they have overwintered I assume they have little in the way of body
reserves left so must forage.

Yes, according to bumblebee.org, the queen must forage heavily to obtain
enough energy to incubate the first generation of brood.

 > When, or indeed do they, eventually change to being fed by the workers?

I'm no bumblebee expert, but according to the literature that I've seen,
the queen ceases foraging once the first generation of workers takes over.
 This is what I also observe in nature.

--
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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